1. Field of the Invention
The present invention relates generally to electrochemical titrators and, more particularly, to titration apparatus for analyzing biological or biochemical substances such as blood serum.
2. Description of the Prior Art
Titrators have long been available for analyzing chloride electrolyte in blood samples by coulometrically generating silver ions which precipitate the chloride as silver chloride and by amperometrically detecting completion of the chloride precipitation. The quantity of silver ions required to precipitate the chloride provides a measure of the amount of chloride initially present in the sample.
Such chloride titrators employ a coulometric generator comprising a pair of coulometric electrodes, typically a large silver anode and a smaller platinum cathode, and an amperometric detector comprising a pair of amperometric electrodes of silver. Both pairs of electrodes are immersed in a reagent solution such as dilute sulfuric acid into which the sample is introduced. When a voltage is applied across the coulometric electrodes, silver ions are generated at the coulometric anode and hydrogen is released at the coulometric cathode. With a small voltage applied across the amperometric electrodes, an amperometric current signal is established therebetween proportional to the quantity of uncombined silver ions present in the solution.
With the system in equilibrium, prior to introduction of a sample into the reagent solution, an initial amperometric current flows between the amperometric electrodes dependent upon the initial quantity of uncombined silver ions present in the solution. The coulometric generator is held off by the initial amperometric current signal and no silver ions are being released into the solution.
When a sample which contains chloride is introduced into the reagent solution, the chloride ions will combine with the uncombined silver ions initially present in the solution to precipitate silver chloride. The resulting decrease in uncombined silver ions results in a corresponding decrease in the amperometric current signal which is coupled to turn on the coulometric generator. Generation of silver ions then proceeds at the coulometric anode until all of the introduced chloride has been precipitated and the initial quantity of uncombined silver ions has been reestablished in the solution. At this time the amperometric current is reestablished at its initial (equilibrium) value, and the coulometric generator is turned off. The total quantity of generated silver ions is measured to provide a measure of the quantity of chloride present in the sample. This may be accomplished by integrating the current flow between the coulometric electrodes.
When protein containing solutions such as blood serum are analyzed in titrators of the type described, the amperometric electrodes and the coulometric cathode (platinum) become encrusted with protein deposits and must be frequently cleaned. When properly cleaned, these electrodes have a relatively long useful life. Cleaning of the coulometric anode (silver) is not generally required since this electrode is consumed by the generation of silver ions and therefore continuously presents a clean exposed surface to the sample.
A major difficulty in arranging the electrodes in the titrator is minimizing electrical and chemical interference between electrodes which would otherwise introduce inaccuracies in the measurement. For example, a fraction of the coulometrically generated silver ions can deposit on the coulometric cathode instead of combining with the sample chloride. Since measurement of the chloride concentration is based on the quantity of silver ions that combine with the chloride ions, if a fraction of the silver ions do not actually combine with chloride ions, then an erroneous chloride measurement is obtained.
In the past, the usual practice has been to mount the amperometric and coulometric electrodes individually in a sample receiving chamber, or at most to mount the electrodes as amperometric or coulometric electrode pairs. Mounting electrodes individually increases the complexity of the titration apparatus. Moreover, maintenance problems are encountered since the electrodes must be individually removed and cleaned.
Often, the electrodes are simply lengths of wire inserted into the top of a laboratory beaker containing the sample to be analyzed. In this form, the electrodes are relatively fragile and subject to damage during cleaning. Moreover, electrode arrangements of this nature are generally unsuited for newer automatic titration apparatus which must rapidly and reliably analyze successive samples for chloride.